Deformable mirrors employ a face sheet or optical membrane carried by a plurality of actuators that can be individually operated to provide micro and macro adjustment of the mirror surface. The optical membrane typically has a plurality of regularly spaced pads on its bonding surface which are bonded to the actuators using a bonding medium such as epoxy. After bonding the mirror surface of the membrane is lapped or polished to obtain a desired mirror finish. Lapping or polishing is typically a wet process using any of a number of different liquids which may be e.g., acid, or alkaline, etching or not-etching. During polishing shims can be used between the membrane and a holding jig to seal the actuators, pads and bonding agent from the liquid. Numerous problems occur: there can still be leakage; the edge of the membrane can be vulnerable to damage by the lap tool. In addition, the shim does not fully support the membrane against the normal force of the lap tool or the shear force of the lap tool as it moves around the mirror surface. These forces can result in a ripple effect in the mirror surface. If the shim is made more stiff or the membrane is edge constrained more rigidly the rippling is reduced but, when the clamping force is released and the membrane relaxes the mirror surface loses the surface accuracy previously achieved and more polishing is necessary. Further, even with the best sealing, some lapping liquid leaks can occur and the absorption of the liquid by the actuator material and the bonding agent causes changes in dimensions. Thus, a membrane which appears to have been polished to the desired finish may, after having dried out after a few hours or days off the grinding machine, lose that finish and have to be polished again and again until the optimum result is obtained.
The lapping action to obtain the desired finish also imposes constraints on the deformable mirror design parameters. While it is desirable to have longer, thinner actuators to produce more compact, high authority, deformable mirrors with a long stroke, the lapping normal and shear forces require a sufficient stiffness of the actuators to withstand those forces without distortion. If the actuators are not stiff enough the finish will be rippled. Typically an actuator aspect ratio of not more than usually 4:1 is used to prevent ripple which consequently constrains the available stroke. With present methods of polishing, a 30 nm surface finish on a 9 or 10 mil membrane is attainable. With such thickness the inter-actuator or relative actuator stroke is limited to approximately ½ the full dynamic range. That is, no actuator should move a stroke distance of more than ½ the full dynamic range relative to its neighboring actuators. If better methods of reacting polishing loads were available then trade offs would be possible. Either higher accuracy finish would be obtainable on the 9 or 10 mil membrane or more traditional finishes could be obtained on much thinner, e.g., 3 mil membranes. Thinner membranes are more flexible and so the limit of inter-actuator or relative stroke for neighboring actuators could be eliminated.